Date:         Mon, 5 Dec 94 19:58:03 PST
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Dave Kautz <dkautz@hpsidms1.sid.hp.com>
Subject:      Re: Tire size question

> David Carment asks: 
>   A simple question for any one with time on their hands...what effect
> does running a larger tire have on the odometer, speedometer and trip
> meter? If for example there are 195s instead of 185s on the front wheels would
> it be necessary
> to increase the distance travelled and speed by say 18% (the difference
> betwen 185s and 195s)? I am curious and also looking for an excuse why my
> mpg is lower than it should be. TIA - DC
> 
> ----------------------------------------------------------------------
> ----------------------------------------------------------------------
> When calculating tire diameter, don't forget to consider the profile ratio.
  The standard size tire for breadloafs and Vanagons is 185R-14. Becuase
  the profile is not specified on the labelling it is therefore 80% as 
  high as it is wide. Add twice the calculated height to the wheel diameter
  and you will get the diameter from which you can calculate circumference
  and so on and so on. By the way, standard wheel and tire dia is just 
  about 25.5 inches. If the profile is specified, like 195/75R-14 or 
  205/70R-14, the number after the / is the percent height to width ratio. I
  think you will find that 205/70R-14 tires are almost identical in diameter
  with the stock tires. What happens on buses and Vanagons all too often is
  someone will put 185/75R-14 tires on them becuase they are more common and
  cheaper. These are actually smaller in diameter causing the speedometer and
  odometer to read optimistically - giving falsely good fuel economy. Tires
  that are larger in diameter will do the opposite, as you surmised.

  Dave Kautz   dkautz@hpsidms1.sid.hp.com     '74 Westfalia